Everything looks exactly the way it should, but as you say, it's not working. I went into your workbook and create a bins on ToPad again (ToPad (bin) 2), then used replaced Padded with the new bins and it works fine. I cannot see any difference between yours and mine, but I think it should work if you just recreate your bins. This is a strange one indeed.

The thing is that now I can produce the sankey chart following the SuperDataScience tutorial, but I'm unable to produce it following Ian's method which seems to be the most efficient way to do it... Quite frustrated.

Anyway, the following is all the tutorials I found on how to create Sankey chart in Tableau so far. Hope it is helpful for others learning this technique:

The purpose of my template was to be plug-and-play--you fill in the data, then plug it into my Tableau template. So, you shouldn't need to create the calculations from scratch. Is that what you've done here or something else?

Usually, building energy balance chart is plotted as a stacked bar chart, in which

Items with positive values indicate sources of heat energy added to building

Items with negative values indicate sources of heat energy removed from building

total heat gains and total heat losses must be the same for each zone

For example, the graph below shows the different types of heat gains or heat losses through different manners for two zones within a building based on the building energy simulation results produced by EnergyPlus.

Since there are negative values, I revised the two calculated fields in your template by calculating the absolute value of [Size]:

Applying your sankey chart template to the data I extracted from the example above, we have the following:

... and a bit tweak on the tooltip to show the energy balance item value related to each thermal zone and the relative ratio: